Method of preparing phosphine



Nov. 5, 1963 l- GORDON METHOD OF PREPARING PHOSPHINE Filed July 27, 1960 m NI CATHODE PHOSPHORUS 3,19,792 METHUD F PREiARING PHUSPHKNE Irving Gordon, Niagara Fails, N.Y., assignor to Hooker Chemical Corporation, Niagara Fails, N.Y., a corporation of New York Filed July 27, 196%,Ser. No. 45,665 14 (liaims. er. ze l-ran This invention relates to the preparation of phosphineby the electrolysis of phosphorus.

Heretofore, phosphine has been prepared by the reaction of metallic phosphides or phosphonium halides with water, and by the hydrolysis of elemental phosphorus. These methods have been unsatisfactory because of the high production costs and/or because the phosphine product is in an impure form.

United States Patent No. 1,375,819, issued April 26th, 1921, to Henry Blumenberg, In, discloses a method for preparing arsine by the electrolysis of a salt or oxide of arsenic in the presence of sulfuric acid and potassium sulfate or other compounds capable of liberating nascent hydrogen upon electrolysis. However, phosphine is not produced under the conditions set forth by Blumenberg when an oxide or salt of phosphorus is employed.

W. R. Grove, in the Journal of the Chemical Society, vol. 16, (1863), pp. 263-272, discloses the use of an electric current to boil moist molten phosphorus and produces phosphine thereby. Such a technique requires a high voltage, and converts only a small amount of phosphorus to phosphine.

It is an object of this invention to provide a method of producing phosphine by electrolytic means.

Another object of the invention is to provide a more economical method of producing phosphine.

Still another object of the invention is to provide a meth- 0d of producing phosphine in a form substantially free from phosphorus hydrides and other phosphorus impurities.

These and other objects of the invention will be apparent from the following detailed description of the invention.

It has been discovered that phosphine can be prepared by passing an electric current between an anode and a cathode in contact with an electrolyte, at least a portion of the cathode being in contact with molten phosphorus, preferably while maintaining the anode free from contact with a molten phosphorus.

The accompanying drawing is a schematic illustration of a typical electrolytic cell suitable for carrying out the novel process.

Referring to the drawing there is shown cell vessel It having a cathode section 11 and an anode section 12, the sections 11 and '12 being separated by a porous diaphragm 13. Gas tight cover 14- having ports 15, 16, 17, 18 and 19, is secured to the top of cell vessel Molten phosphorus 21 is contained in the bottom portion of cathode section 11, the upper level being indicated by inter-face 21. An electrolyte Z2, is contained in the cathode section 11 and anode section 12, the upper level of the electrolyte being indicated by electrolyte interface 23. Cathode 24 extends through port 15 and electrolyte 22 into the molten phosphorus 20. Cathode '24, is shown here as a solid plate, but other forms of cathode can be employed as will be discussed more fully hereinafter.

Anode 25 extends through port 16 into the electrolyte 22 contained in anode section 12. Electric conductors 26 and 27 connect the anode 25 and cathode 2.4, respectively, to the positive and negative poles, respectively, of a source of electrical energy 28.

When an electric current is impressed upon the system a phosphine-containing gas is generated in the cathode section 11 and is discharged through catholyte gas discharge rates Patent ice 2 line 29, which extends through port 17. At the same time the anolyte gas formed in the anode section :12 is discharged through anolyte gas discharge line 31 which extends through port 18.

A fresh supply of molten phosphorus and/ or electrolyte may be introduced into cathode section 11 by means of funnel 32 which passes from the cell vessel exterior through port 19 into cathode section 11. If desired, a motor driven impeller 33, or other suitable agitation means, may be positioned in the bottom portion of cathode section 11 to efiect agitation of the molten phosphorus.

It will be recognized by those skilled in the art that the design of the electrolytic cell shown in the drawing may be modified without departing from the spirit of the invention. For example, the cathode is shown in the drawing as a solid plate, but a liquid cathode such as mercury may be employed. In such a case liquid mercury is placed in the bottom of cathode section 111 below the molten phosphorus, and an electrical conductor 27 is extended through port 15 into the liquid cathode at the bottom. of cathode section 11. In this case, it is necessary to agitate both the liquid cathode and the molten phosphorus in order that the liquid cathode contacts not only the molten phosphorus but also the electrolyte, thereby permitting the current to pass between the cathode and anode.

The cell may be further modified when graphite is employed as the anode. Under these conditions, a gaseous mixture of phosphine and hydrogen form at the cathode and carbon dioxide forms at the anode. If desired, diaphragm 13 may be removed from cell vessel 10, and molten phosphorus may be distributed over the bottom portion of the cell vessel. The catholyte gas and anolyte gas can be combined in this instance, and employed as a reactant where the catholyte gas is employed, since the carbon dioxide component of the mixture is inert. However, if it is desired to remove the carbon dioxide, the mixed catholyte-anolyte gas may be passed through caustic solution to separate carbon dioxide from the gas mixture. Since the catholyte-anolyte gaseous mixture is substantially free from oxygen, there is no danger of oxidizing or burning the phosphorus in the cell, and there is no danger of oxidizing or burning the phosphine gas within or without the cell.

The cell vessel may be constructed of any impervious material such as glass, ceramics, rubber-lined steel and the like.

Diaphragm 13 may be constructed of any suitable porous material such as sintered glass, porous Alundum, ionexchange membranes, plastic cloth, glass cloth and the like.

Any material having a hydrogen overvoltage as normally measured in the absence of phosphorus exceeding the hydrogen overvoltage of smooth platinum may be employed as the cathode. Typical cathodic materials include lead, lead-mercury amalgam, tin, mercury, cadmium, copper, bismuth, aluminum, zinc, brass, silver, nickel, tellurium, monel, gold, and alloys thereof. For example, the alloy known as Woods metal, which is an alloy containing fifty percent bismuth, twenty-five percent lead, twelve and one-half percent tin, and twelve and one-half percent cadmium, may be employed. This alloy may be used in either liquid or solid form. Black phosphorus may also be employed as a cathode material.

It is desirable to employ a cathode in a form having a high unit of area per unit of weight. As indicated previously the drawing shows the cathode in the form of a solid plate. If desired, when a solid cathode is employed, the cathode may have the form of a helical coil, wire gauze or screen, perforated sheets, and the like.

Suitable anode materials include lead, lead-antimony, lead dioxide, platinum, graphite and stainless steel.

Any electrolyte which is non-reactive with molten phosphorus, and which is capable of forming hydrogen ions under the electrolysis conditions employed, may be employed as the electrolyte.

Suitable electrolytes include aqueous solutions of phosphoric acid, sulfuric acid, hydrochloric acid, sodium chloride, lithium chloride, sodium sulfate, potassium chloride, potassium sulfate, monosodium phosphate, disodium phosphate, and mixtures thereof. An aqueous phos phoric acid solution containing between about ten and about eighty, and preferably between about fifteen. and about fifty percent phosphoric acid by weight, is preferably employed as the electrolyte, but other concentrationsmay be employed if desired. Concentration of the aqueous solutions of the aforesaid acids, when em ployed as an electrolyte, should be equivalent to the afore said phosphoric acid concentration range. Aqueous solutions of the aforesaid salts having a concentration between about ten percent by .Ieight and the concentration sufficient to produce a saturated solution under the temperature conditions obtained, may be used.

If phosphoric acid, sulfuric acid, or salts of these acids are employed as the electrolyte, the anolyte gas predominates in oxygen, when other than graphite is employed as the anode. If these acids and/or salts are employed as the electrolyte, and the anode is constructed of graphite, the anolyte gas will predominate in carbon dioxide.

Molten White phosphorus, sometimes referred to as yellow phosphorus, is preferably employed as the source of phosphorus for the production of phosphine, but other allotropic forms of phosphorus may be employed if desired. The temperature of the phosphorus should be suflicient to maintain it in a molten state, without alfeoting boiling hereof. For this reason, the temperature of the molten phosphorus and electrolyte is maintained within the range between about forty-four degrees and about two hundred and eighty degrees centigrade, and preferably between about fifty and about one hundred and twenty degrees centigrade. Temperature control of the phosphorus and .the electrolyte may be readily obtained by means of constant temperature bath (not shown in the drawing) surrounding cell vessel ill, but any suitable temperature control means may be employed. For example, on start-up of the electrolytic process the molten phosphorus and electrolyte may be heated to a tempera ture within the aforesaid temperature range by means of an external source of heat, and maintained at this temperature by means of a constant temperature bath.

When a solid cathode is employed in the process, a wiching effect occurs between the surface of the solid cathode and the molten phosphorus, the molten phosphorus adhering to the surface of the solid cathode and rising above molten phosphorus interface 21. This wicking effect occurs in some instances before a current is impressed upon the system, but in all cases occurs when an electric current is impressed upon the system. Agitation of the molten phosphorus enhances this wicking effect. 7

The rate of phosphine production and the purity of the phosphine product varies with the current and cur rent density. When a low current density is employed, a gaseous mixture of phosphine and hydrogen is produced at the cathode, the resulting gas mixture containing a high concentration of phosphine. However, under these conditions, the production rate of the gas mixture is relatively low, being generally below the level which is considered economically feasible. When high current densities are employed the production rate of the phosphine-containing gas is increased, but the concentration of phosphine is reduced. increasing the cathodic current density will increase the production rate, but also reduces the phosphine concentration in the catholyte gas. It will be recognized by those skilled in the art that the optimum current and optimum current density will vary with the size and design of the cell, but each case it is important to employ those conditions that yield a catliolyte gas having a high concentration of phosphine consistent with commercially feasible production rates. For example, current density within the range between about five and about one thousand amperes per square foot yield optimum results in most instances, but higher or lower densities may be employed if desired. It is important to employ, in each case, current which will produce a voltage drop across the system of less than about twenty-five volts and preferably less than about ten volts. When the voltage drop is in excess of about twenty-five volts, at significant proportion of the electrical energy imparted to the cell is wasted in heating the ingredients in the cell, instead of being utilized to effect electrolytic decomposition thereof.

The catholyte gas produced in accordance with the instant novel process is a mixture of phosphine and hydrogen, containing as high as ninety percent phosphine or higher. An important advantage of this process is that the phosphine-containing gas is relatively free of other phosphorus hydrides compared to phosphine produced by prior art techniques, and as a result the gas is not spontaneously flammable when contacted with air.

When an acid is used as the electrolyte, the resulting catholyte gas is substantially free from phosphorus hydride impurities. As a result when the catholyte gas is employed as a chemical intermediate, the product of the reaction is in a highly pure form. For example, tetra'kis (hydroxymethyl) phosphonium chloride is prepared by the reaction of phosphine, formaldehyde, and concentrated hydrochloric acid. When phosphine prepared by conventional processes is employed to produce tetrakis (hydroxymethyl) phosphonium chloride, the resulting product has a purity of about 96.5 percent. In contrast, when p'hosphine prepared in accordance with the instant novel process is used to prepare tetrakis (hydroxymethyl) phosphonium chloride, the purity of the product is as high as 99.9 percent by weight.

The following examples are presented to define the invention more clearly without any intention of being limited thereby. All parts and percentages are by weight unless otherwise specified.

Example 1 An electrolytic cell was constructed as follows: A five hundred milliliter glass beaker having a gas-tight rubber stopper secured to the top was employed as the cell vessel. A porous sintered glass cylinder was inserted into the cell vessel through the rubber stopper to a point adjacent to the bottom of the cell vessel. The porous sintered glass served as a diaphragm to separate the cathode section (the volume outside of the sintered glass cylinder) from the anode section (the volume inside the sintered glass cylinder).

A lead disk, which was about one-quarter inch thick and about three inches in diameter was positioned in the cathode section as the cathode. A threecighths inch diameter graphite rod, which was approximately six inches long, was inserted into the anode section to serve as the anode. Copper Wire connected the graphite rod and the lead disk to a source of direct current. Glass tubing passed through the top of the cell vessel served as a means of removing catholyte gas, and glass tubing inserted in the gas-tight cover of the sintered glass cylinder served to remove the anolyte gas.

Fifty-three grams of molten phosphorus was placed in the cathode section of the cell vessel. Four hundred milliliters of an aqueous eighteen percent phosphoric acid solution was added to the cathode and anode sections to serve as an electrolyte. The lead disk cathode contacted the molten phosphorus as well as the electrolyte. A gitation of the electrolyte and molten phosphorus was effected by means of a plastic coated magnetic stirrer which was rotated at the rate of about two hundred revolutions per minute.

A current of one ampere and a voltage of thirteen volts was impressed upon the system during electrolysis. The temperature of the electrolyte and molten phosphorus was maintained at about forty-six degrees centigrade by placing the cell vessel in a constant temperature bath.

The gaseous mixture of phosphine and hydrogen produced at the cathode had a phosphine concentration of fifteen percent by volume and was produced at the rate of five milliliters per minute.

Example .2

The procedure of Example 1, employing the cell of Example 1, was repeated with the exceptions noted hereinafter. A Woods metal disk replaced the lead disk as a cathode. Thirty grams of molten phosphorus was placed in the cathode section of the cell vessel. A current of two amperes and a voltage of 17.5 volts was impressed upon the system during electrolysis. The temperature of the molten phosphorus and electrolyte was maintained at about forty-five degrees centigrade during electrolysis.

The catholyte gas contained 4.5 percent phosphine by Volume and was produced at the rate of 3.9 milliliters per minute.

Example 3 The procedure of Example 1, employing the cell of Example 1, was repeated, with the exceptions noted hereinafter. A cadmium disk replaced the lead disk as the cathode. Thirty grams of molten phosphorus was added to the cathode section of the cell vessel. About four hundred and fifty milliliters of an aqueous eighteen percent phosphoric acid solution was added to the cathode and anode sections to serve as an electrolyte.

A current of two amperes and a voltage of twentythree volts was impressed upon the system during the electrolysis. The molten phosphorus and electrolyte were maintained at a temperature of eighty degrees centigrade during electrolysis.

The catholyte gas was produced at the rate of 12.2 milliliters per minute and contained 39.5 percent phosphine by volume.

Example 4 The procedure of Example 1, employing the cell design of Example 1, was repeated with the exceptions noted hereinafter. The cathode was a helical coil of one-eighth inch diameter lead wire, the coil having a height of six inches, a diameter of two and one-half inches, and an effective cathodic area of 43.2 square inches. The anode was a platinum gauze cylinder two inches high, one-half inch in diameter, and having an effective anodic area of about 3.14 square inches.

One hundred grams of molten phosphorus were added to the cathode section of the cell vessel and three hundred and fifty milliliters of electrolyte were added to the cathode and anode sections.

A current of three amperes and a voltage of 17.2 volts was impressed upon the system during electrolysis. The temperature of the molten phosphorus and electrolyte was maintained at about eighty-five degrees centigrade during electrolysis.

The catholyte gas had a phosphorus concentration of about sixty percent by volume and was produced at the rate of 17.9 milliliters per minute.

Example 5 A cell was constructed as follows: Into a three-liter beaker was placed an inverted beaker having seven openings through the inverted bottom. A first opening received electrolyte temperature recording means. A second opening received phosphorus temperature recording means. A third opening received the cathode current lead wire. A fourth opening received the initial nitrogen purge gas and later the molten phosphorus and elec trolyte which Were fed in periodically. Openings five and six were spaced on either side of said third opening and served to receive each of the lead wires to two anodes, one on either side of the cathode, and also to serve as outlets for the anode compartment. gases (oxygen). The seventh opening served as an outlet for the cathode compartment gases (PH and H Each of the anode compartments were constructed of glass containing an anode of porous lead peroxide with the side toward the cathode faced with a semi-porous: diaphragm. The cathode was a sheet of commercial porous lead. The phosphorus was molten commercial yellow phosphorus and the electrolyte was a forty percent solution of H PO In starting up the cell, the phosphoric acid was introduced into the cell. Then the current was turned on to make the cathode cathodic, which reduced oxides on the surface of the cathode. Next, the cell was purged with nitrogen, and five hundred and three grams of molten yellow phosphorus were added to :cover the bottom of the cell and to contact the bottom edge of the cathode sheet.

Again the current was turned on. Oxygen was produced at the anode and phosphine began to be produced With hydrogen at the cathode. The phosphorus immediately began to wick up both sides of the cathode to the top, to a height of five and one-half inches. The dividing line on the cathode between the wicked and unwicked portion could be identified by the change in the appearance of the bubbles coming off the surfaces and also by the physical appearance of the surfaces. After a few hours the surfaces of the cathode were totally covered with phosphorus. The cathode gas analysis gradually enriched in PHg, and after about twenty-four hours of operation reached a maximum value of eighty percent PH by volume, at 2.9 volts and current density of 6.0 amperes per square foot at about one hundred degrees centigrade.

Example 6 The procedure of Example 1, employing the cell of Example 1, was repeated, with the exceptions noted hereinafter. Fifty milliliters of mercury were added to the cathode section to serve as a cathode instead of the lead disk. A copper wire immersed in the mercury connected the cathode with the source of direct current. Thirty grams of molten phosphorus were added to the cathode section. A current of one-half ampere, and a voltage of eight volts was impressed upon the system during electro ysis. A temperature of ninety degrees centigrade was maintained in the cathode section.

The gaseous mixture of phosphine and hydrogen produced at the cathode contained ninety-five percent phosphine by volume, and Was produced at the rate of 2.7 milliliters per minute.

It will be recognized by those skilled in the art that various modifications within the invention are possible, some of which have been referred to above. Therefore, I do not wish to be limited except as defined by the appended claims.

I claim:

1. The process for preparing phosphine which comprises contacting an anode and a cathode with an aqueous inorganic electrolyte, at least a portion of the cathodic surface of said cathode containing elemental phosphorus, and passing an electric current between said anode and said cathode through said electrolyte, to yield a cathodic current density during electrolysis in the range between about five and about one thousand amperes per square foot, where-by a phosphine-containing gas is produced at the cathode.

2. The process for preparing phosphine which comprises contacting an anode and a solid cathode with an aqueous inorganic electrolyte, contacting a portion of said cathode with molten phosphorus, and passing .an electric current between said anode and said cathode through said 7 electrolyte, to yield a cathodic current density during electrolysis in the range between about five and about one thousand amperes per square foot, whereby a phosphinecontaining gas is produced at the cathode.

3. The process for preparing phosphine which comprises contacting an anode and a solid cathode with an aqueous inorganic electrolyte, contacting a portion of said cathode with molten phosphorus while maintaining sm'd anode free from contact with said molten phosphorus, and

passing an electric current between said anode and said cathode through said electrolyte to yield a cathodic current density during electrolysis in the range between about five and about one thousand 'amperes per square foot, whereby a phosphine-containing gas is produced at the cathode.

4. The process of claim 1 wherein said cathode is a solid material having a hydrogen overvoltage as normally measured in the absence of phosphorus exceeding the hydrogen overvoltage of smooth platinum.

5. The process of claim 1 wherein said can ode is lead.

6. The process of claim 1 wherein said cathode is cadmium.

7. The process of claim 1 wherein said cathode is mercury.

8. The process of claim 1 wherein said cathode is an alloy containing bismuth, tin, lead and cadmium.

9. The process of claim 1 wherein said electrolyte is non-reactive with molten phosphorus, and is capable of forming hydrogen ions under the electrolysis conditions employed.

10. The process of claim 1 wherein said electrolyte is an aqueous solution of phosphoric acid.

' 11. The process of claim 1 wherein said electrolyte is an aqueous solution of phosphoric acid containing b tween about fifteen and about fifty percent phosphoric acid by Weight.

12. The process of claim 2 wherein the temperature of the electrolyte and molten phosphorus is maintained dur- 8 ing electrolysis within the range between about fifty and about one hundred and twenty degrees centrigrade.

' 13. The process of claim 1 wherein the voltage during electrolysis is maintained below about ten volts.

14. The process for preparing phosphine which comprises passing an electric current between a cathode and an anode in contact with an aqueous phosphoric acid solulation electrolyte, at least a portion of said cathode being in contact with molten phosphorus, said cathode having a hydrogen overvoltage as normally measured in the absence of phosphorus exceeding the hydrogen over-voltage of smooth platinum, the cathodic current density during electrolysis being maintained between the five and about one thousand amperes per square foot, and the voltage drop across the system being maintained below about twenty-five volts, whereby a phosphine-cont-aining gas is produced in the cathode.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Ephraim: inorganic Chemistry, 5th ed. (1948), pages 617-22.

Pauling: College Chemistry (1955), pages 330-5.

Journal of Chemical Society, volume 16 (1863), pages 263-72. 

1. THE PROCESS FOR PREPARING PHOSPHINE WHICH COMPRISES CONTACTING AN ANODE AND A CATHODE WITH AN AQUEOUS INORGANIC ELECTROLYTE, AT LEAST A PORTON OF THE CATHODIC SURFACE OF SAID CATHODE CONTAINING ELEMENTAL PHOSPHORUS, AND PASSING AN ELECTRIC CURRENT BETWEEN SAID ANODE AND SAID CATHODE THROUGH SAID ELECTROLYTE, TO YIELD A CATHODIC CURRENT DENSITY DURING ELECTROLYSIS IN THE RANGE BETWEEN ABOUT FIVE AND ABOUT ONE THOUSAND AMPERES PER SQUARE FOOT, WHEREBY A PHOSPHINE-CONTAINING GAS IS PRODUCED AT THE CATHODE. 